Melt the butter in a large saucepan, then add the onions. SLOWLY brown the onions. Seriously. Should take about an hour. They should be soft, golden brown and transparent. Add the garlic after the onions have been cooking for about 45 minutes.

After the onions are cooked, turn up the heat to medium. Stir in the flour and cook until golden brown (about a minute).

Add the stock and stir until it boils. Season with salt and pepper and simmer for about 20-30 minutes.

Preheat the oven to 400 F. Mix the grated cheese with the mustard and pepper. Spread this mix on the bread slices and place each slice on the bottom of a bowl (or all in a big soup tureen). Pour the soup - the bread will float to the top.

Bake the soup in the oven, uncovered, until the bread and cheese mixture is well browned and bubbling.